Remote alarm reporting systems are known in which the monitoring and intelligence of the system is located in a central location, remote from the area that is monitored. Sensor inputs located in the monitored area can be coupled to the monitor via telephone lines, for example. These sensors can be transducers which measure pressure, temperature, etc. The transducers are pneumatic or cathodic, and are always resistive.
The transducers are located at the monitored area across the tip and ring conductors of a subscriber's telephone lines. The transducers are connected to what is known as a "line card", such as a GTD 5A line card (manufactured by GTE), through a combination of blocking diodes. The intelligence of the system, which can be microprocessor based for example, monitors the condition of the transducers and will respond to alarm conditions sensed by the transducers by dialing a preconfigured telephone number.
The measurement of the resistance of some transducers is made difficult by the fact that the transducer is always powered by a special line card such as the GTD 5A line card through the blocking diodes. This is known as a "wet" measurement, since there is voltage on the lines. This measuring is made even more difficult since the GTD 5A line card contains a symmetric, but variable voltage on the tip and ring conductors. In addition, this variable voltage varies with the load on the transducer, cable shorts and the subscriber's phone offhook across the tip and ring.
A device is known which will make wet measurements. It does so by placing pre-set voltages on the tip and ring conductors so that there is no current flow through the blocking diodes. For example, assume that the conductor at the line card that is connected to the tip conductor is at 0 volts, and the conductor at this line card that is connected to the ring conductor is at (-)48 volts. The known device will make a wet measurement by first placing an equal voltage of 0 volts on the tip conductor on the other side of the blocking diodes, between the blocking diodes and the transducers. Since the known device places a preset voltage on the tip conductor, it does not account for the variable voltage (a bias voltage) that the GTD 5A line card contains. This bias voltage varies from line card to line card, and also in dependence on the load. The bias voltage ranges from, for example, (-)2 to (-)6 volts. This failure to account for the bias voltage due to different line cards and the load leads to inaccurate wet measurements of the resistance of the transducers.
There is a need for a method and apparatus for making measurements of transducers that are coupled by a metallic pair to a line card, which accounts for variable bias voltages, to produce an accurate wet measurement.